Actually, you don't have to report this anywhere on your US tax return, or pay any US tax on it. The fact that you experienced a currency gain on the mortgage refinancing, if you will, is incidental to the refinancing process of an overseas property.
If, instead, you had been trading or speculating directly on foreign currencies or currency contracts, such as the US dollar / Singapore dollar exchange cross, then that would be a reportable (and taxable) gain or loss. However, in this instance, that doesn't apply. Similarly, if you had experienced a net currency loss on the mortgage refinancing, in lieu of a gain, then that wouldn't be a tax deducible event either.
Where US tax reporting is applicable in this matter, is just where you would otherwise expect it to be. First, there is the potential to deduct mortgage interest if this is your first or second home. Second, if and when you sell your Singapore property, you may have a capital gain (the US taxes on which may be mitigated or eliminated by an IRC Section 121 exclusion). There are some links provided below that will take you to where you can read more about both of these topics.
https://turbotax.intuit.com/tax-tools/tax-tips/Home-Ownership/Deducting-Mortgage-Interest-FAQs/INF12...
https://www.irs.gov/taxtopics/tc701.html
Thank you for asking this question.
